Resistance spot welding is a process used in a number of industries to join together two or more metal workpieces. The automotive industry, for instance, often uses resistance spot welding to join together metal workpieces during the manufacture of a vehicle door, hood, trunk lid, or lift gate, among other vehicle components. Multiple resistance spot welds are typically made along a periphery of the metal workpieces or at some other location. While spot welding has typically been performed to join together certain similarly-composed metal layers—such as steel-to-steel and aluminum alloy-to-aluminum alloy—the desire to incorporate lighter weight materials into a vehicle platform has created interest in joining steel workpieces to aluminum or aluminum alloy (hereafter collectively “aluminum” for brevity) workpieces by resistance spot welding. Moreover, the ability to resistance spot weld workpiece stack-ups containing different workpiece combinations (e.g., aluminum/aluminum, steel/steel, and aluminum/steel) with one piece of equipment would increase production flexibility and reduce manufacturing costs.
Resistance spot welding, in general, relies on the resistance to the flow of electric current through contacting metal workpieces and across their faying interface to generate heat. To carry out a resistance welding process, a pair of opposed welding electrodes is clamped at aligned spots on opposite sides of the metal workpieces at a weld site. An electrical current is then passed through the workpieces from one welding electrode to the other. Resistance to the flow of the electric current generates heat within the metal workpieces and at their faying interface. When the workpieces being spot welded are a steel workpiece and an aluminum workpiece, the heat generated at the faying interface typically initiates a molten weld pool that penetrates into the aluminum workpieces from the faying interface. The molten weld pool wets the adjacent surface of the steel workpiece and, upon cessation of the current flow, solidifies into a weld nugget that forms all or part of a weld joint. After the spot welding process is completed, the welding electrodes are retracted from the workpiece surfaces and the spot welding process is repeated at another weld site.
Resistance spot welding a steel and an aluminum workpiece, however, can be challenging since the two metals possess different properties that tend to complicate the welding process. Specifically, steel has a relatively high melting point (˜1500° C.) and relatively high electrical and thermal resistivities, while aluminum has a relatively low melting point (˜600° C.) and relatively low electrical and thermal resistivities. As a result, most of the heat is generated in the steel workpiece during electrical current flow. This heat imbalance sets up a temperature gradient between the steel workpiece (higher temperature) and the aluminum workpiece (lower temperature) that initiates rapid melting of the aluminum workpiece. The combination of the temperature gradient created during current flow and the high thermal conductivity of the aluminum workpiece means that, immediately after the electrical current has ceased, a situation occurs where heat is not disseminated symmetrically from the weld site. Instead, heat is conducted from the hotter steel workpiece through the aluminum workpiece towards the welding electrode in contact with the aluminum workpiece, creating relatively steep thermal gradients in that direction.
The development of steep thermal gradients between the steel workpiece and the welding electrode in contact with the aluminum workpiece is believed to weaken the integrity of the resultant weld joint in two primary ways. First, because the steel workpiece retains heat for a longer duration than the aluminum workpiece after the electrical current has ceased, the molten weld pool that has been initiated and grown in the aluminum workpiece solidifies directionally, starting from the region nearest the colder welding electrode (often water cooled) associated with the aluminum workpiece and propagating towards the faying interface. A solidification front of this kind tends to sweep or drive defects—such as gas porosity, shrinkage voids, micro-cracking, and oxide residue—towards and along the faying interface within the aluminum weld nugget. Second, a sustained elevated temperature in the steel workpiece promotes the growth of brittle Fe—Al intermetallic compounds at and along the faying interface. The intermetallic compounds tend to form thin reaction layers between the aluminum weld nugget and the steel workpiece. If present, these intermetallic layers are generally considered part of the weld joint along with the weld nugget. Having a dispersion of weld nugget defects together with excessive growth of Fe—Al intermetallic compounds along the faying interface is thought to reduce the peel strength of the final weld joint.